1. Technical Field
The present invention relates to a photomask and a semiconductor device.
2. Description of Related Art
In a manufacturing method of a semiconductor device, there are known a photolithography process and an etching process as a method for forming a desired pattern on a processed film such as an insulating film or a conductive film on a semiconductor substrate. The photolithography process includes the exposure process and development process. An exposure device used for the exposure process is briefly described.
FIG. 1 is a sectional view schematically showing a configuration example of the exposure device that uses the related photomask. As shown in FIG. 1, the exposure device includes light source 1, condenser lens 2, photomask 3, reduced projection lens 4, and stage 6. An argon-fluoride (ArF) laser is used for light source 1. A wavelength of the light source, which is not limited to that of the ArF laser, can be a wavelength longer or shorter than that of the ArF laser.
Photomask 3, which is located between condenser lens 2 and reduced projection lens 4, is changed according to a pattern formed on the processed film. In photomask 3, a pattern is formed on a light shielding film of chromium (Cr) on a glass substrate. In this case, photomask 3 is presumed to be a size-increased mask having a lager size than the real size of a pattern projected to wafer 5. This size-increased mask is referred to as a “reticle”. A certain type of a reticle has patterns equivalent to a plurality of chips formed on a glass substrate of one reticle.
Wafer 5, which has the processed film and a resist film stacked on the semiconductor substrate, is mounted on stage 6. Stage 6 can move wafer 5 in each of a horizontal direction (X direction) and a depth direction (Y direction) shown by a driving unit (not shown).
Next, the exposure process using the exposure device shown in FIG. 1 is briefly described. It is presumed that photomask 3 has first been set in the exposure device and then wafer 5 having a photosensitive polymer uniformly applied as a photoresist on the surface has been mounted on stage 6.
As shown in FIG. 1, light emitted from light source 1 is adjusted to be almost vertical during passage through condenser lens 2 to be applied to photomask 3. The light transmitted through photomask 3 is set to an intensity according to the brightness and darkness of photomask 3, reduced by reduced projection lens 4, and then applied to wafer 5, thereby forming a pattern on the photoresist according to the light intensity. Each time light irradiation process is performed to irradiate the photoresist with light via photomask 3 and reduced projection lens 4 for a given period of time, stage 6 on which wafer 5 is supported is moved in one of the X direction and the Y direction, and the light irradiation process is repeated. The photoresist is thoroughly irradiated with the light so as to prevent overlapping of areas irradiated with the light. This enables formation of a plurality of patterns on the entire photoresist according to the light intensity.
Photosensitive components contained in the photoresist vary in reaction speed from one light intensity to another for pattern formation on the photoresist. Hence, in the development process after the exposure process, the patterns of photomask 3 are transferred to the photoresist by using a difference, between the reaction speeds, in solubility of the photoresist in a developer. Thus, the patterns of photomask 3 are exposed with the light intensity based on the brightness and darkness of photoresist 3 to be transferred to wafer 5.
Next, as a specific example of exposure process, a method for transferring a line pattern and a plurality of dot patterns to the photoresist is described. The transferring status of the patterns of photomask 3 to the photoresist is described in detail referring to the expanded view of broken-line part 7 and broken-line part 8 of photomask 3 shown in FIG. 3.
FIGS. 2A to 2C are explanatory views showing a pattern forming method that uses the related photomask.
FIG. 2A shows a partial configuration of related photomask 500: an upper side being a plan view of photomask 500, and a lower side being a sectional view of the AA part shown in the plan view. FIG. 2B is a graph showing an intensity distribution when the light from the light source is transmitted through photomask 500 shown in FIG. 2A and the optimal focus is set on the surface of the photoresist. FIG. 2C shows a pattern shape of the photoresist after development when exposure is carried out with the light intensity shown in FIG. 2B: an upper side being a plan view, and a lower side being a sectional view of a BB part shown in the plan view.
As shown in FIG. 2A, in photomask 500, line pattern 10 is disposed on the rear surface of substrate 9 to extend in the Y direction, and a plurality of dot patterns are arranged in a grid pattern on the rear surface of substrate 9 in the X and Y directions. An area in which the plurality of dot patterns are arranged in the grid pattern in the X and Y directions is denoted by reference numeral 11. Line pattern 10 and a plurality of dot patterns are formed by light shielding films in which chromium and a chromium oxide (CrOx) are stacked.
Hereinafter, for simpler description, the plurality of dot patterns arranged in photomask 500 are divided into a plurality of groups, a plurality of patterns arranged at equal intervals in the Y direction constituting one group, and the patterns of each group is referred to as “dot patterns”. FIG. 2a shows dot pattern 11a and dot pattern 11b. 
Dot pattern 11a closest to line pattern 10 is located away from line pattern 10 by space 12, and away from adjacent dot pattern 11b by space 13. Space 12 has width X1 in the X direction, and space 13 has width X2 in the X direction.
For convenience, widths that are lengths of line pattern 10 and dot patterns 11a and 11b in the X direction are set equal. Width X1 is larger than width X2, and the respective patterns are arranged so that space 12 between line pattern 10 and dot pattern 11a can be larger than space 13 between adjacent dot patterns 11a and 11b. Though not shown, a pattern identical to line pattern 10 is disposed on the left side of line pattern 10 via a space having width X2, and a pattern identical to dot pattern 11b is disposed on the right side of dot pattern 11b via a space having width X2.
In light intensity graph 200 shown in FIG. 2B, a horizontal axis indicates positions of the patterns and the spaces of photomask 500, and a virtual axis indicates light intensity. Light intensity is plotted on the photoresist surface according to the position of each of the patterns and the spaces.
As shown in FIG. 2B, the light intensity greatly varies from one position to another of the patterns and the spaces, and becomes minimum value 14 at the center of each of line pattern 10 and dot patterns 11a and 11b in photomask 500. The intensity value, which is set to minimum value 14 at these positions, never becomes zero. This is because the patterns of photomask 500 block the light from the light source, and a part of the light transmitted through spaces 12 and 13 becomes diffracted light due to a diffraction phenomenon, and moves around to the lower surface of each pattern to reach the resist surface.
A diffraction angle θ with a vertical direction set as a base point is represented by a relational expression of sin θ=nλ/pitch, where λ is a wavelength of the light emitted from the light source, the pitch is a sum total of widths of the patterns and the spaces, and n is an index (n=±1, ±2, . . . ) indicating an order (first-order diffraction, second-order diffraction, . . . ) of the diffracted light. For example, one pitch of dot pattern 11a is represented by “width of dot pattern 11a+width X2 of space 13”.
It can be understood from the relational expression that as the space becomes wider, the diffraction angle θ becomes smaller. In photomask 500 in which the patterns are formed with their widths set equal, as the wider is the space, the greater is the amount of light that moves around to the lower surface of each pattern, thereby increasing the amount of light to contribute to the exposure. Thus, as shown in FIG. 2B, light intensity 15 at the end of line pattern 10 facing space 12 which is wider than space 13, is larger by 1 than light intensity 16 at the opposite end. Similarly, light intensity 17 at the end of dot pattern 11a facing space 12, is wider by 2 than light intensity 18 at the opposite end.
However, light intensities 19 and 20 at the end of dot pattern 11b that faces narrow space 13 are stable at values almost equal to that of light intensity 18. The light intensity takes the maximum value at the center of the space. However, because of the abovementioned diffraction phenomenon, the light is greatly scattered in narrow space 13 to reduce the amount of light reaching the photoresist. Thus, maximum value 21 between dot pattern 11a and dot pattern 11b is smaller than maximum value 22 between line pattern 10 and dot pattern 11a. In FIG. 2B, threshold value 23 of the light intensity in the development process is indicated by a broken line and, in an area of the light intensity larger than threshold value 23, a positive photoresist is dissolved during the development process.
The light intensity accordingly varies depending on the arrangement of the patterns and the spaces. The light intensity greatly fluctuates especially in a peripheral part where the pattern density greatly differs in an area in which pluralities of identical patterns are arranged.
FIG. 2C shows a pattern shape of the photoresist after the development when the exposure is carried out with the light intensity shown in FIG. 2B. The photoresist is a positive type. As shown in FIG. 2C, processed film 25 is formed on semiconductor substrate 24 that is a part of the wafer, and the photoresist that becomes a processed mask of processed film 25 is formed on processed film 25. Then, the patterns of photomask 500 are transferred to the photoresist. Hereinafter, a pattern in which a line pattern is transferred to the photoresist is referred to as a line resist, and a pattern in which a dot pattern is transferred to the photoresist is referred to as a dot resist.
The patterns of the photoresist that becomes the processed mask are largely classified into line resist 26 and dot resist 27. Dot resist 27 is further classified into dot resist 27a adjacent to line resist 26 via space 28, and dot resist 27b adjacent to dot resist 27a via space 29.
In this case, since the photoresist is the positive type, the photoresist at the exposure place is removed by the development process to form spaces 28 and 29. Light intensity at the end facing space 28 increases due to diffracted light, and hence line resist 26 is formed smaller by width X3 than virtual end 30 when there is no diffracted light. Similarly, dot resist 27a facing space 28 is formed smaller by width X4 than virtual end 31. On the other hand, the longest part of dot resist 27b in the X direction is formed with a width almost equal to that of dot pattern 11b shown in FIG. 2A.
Thus, when the patterns of the peripheral part of the area in which the plurality of identical patterns are formed are formed smaller than the desired sizes, patterns in which these patterns are transferred to the processed film are also formed smaller than the desired shapes. As a result, the patterns formed small are easily peeled during the manufacturing process. When pattern peeling occurs, the problem of reduction in product yield is created.
The case where the exposure process is carried out with the optimal focus has been described referring to FIGS. 2A to 2C. A case where defocusing occurs is described referring to FIGS. 3A to 3C. The defocusing means a state where an out-of focus state is set to cause a formed image to blur.
FIG. 3A shows a partial configuration of related photomask 500: an upper side being a plan view of photomask 500, and a lower side being a sectional view of a CC part shown in the plan view. FIG. 3B is a graph showing an intensity distribution when the light from the light source is transmitted through photomask 500 shown in FIG. 3A and defocusing occurs on the surface of the photoresist. FIG. 3C shows a pattern shape of the photoresist when exposure is carried out with the light intensity shown in FIG. 3B: an upper side being a plan view, and a lower side being a sectional view of a DD part shown in the plan view.
Photomask 500 shown in FIG. 3A is similar in configuration to photomask 500 shown in FIG. 2A. To avoid repeated description, description of components similar to those shown in FIG. 2A is omitted.
In light intensity graph 210 shown in FIG. 3B, a horizontal axis indicates positions of the patterns and the spaces of photomask 500, and a virtual axis indicates light intensity. Light intensity is plotted on the photoresist surface according to the position of each of the patterns and the spaces. For comparison, light intensity graph 200 at the time of the optimal focus state shown in FIG. 2B is indicated by a broken line, and threshold value 33 equal to threshold value 23 is indicated by a broken line.
In light intensity graph 210, the light intensity at the time of the defocusing state, which has amplitude (difference between maximum value and minimum value) smaller than that at the time of the optimal focus state, greatly varies from one position to another of the patterns and the spaces, and becomes minimum value 32 at the center of each of line pattern 10 and dot patterns 11a and 11b in photomask 500. Minimum values 32a and 32b closest to wide space 12 are larger than minimum values 14a and 14b at the time of the optimal focus state, and are shifted to the inside of each pattern. Light intensity 34 at threshold value 33 is accordingly shifted more to the inside of each pattern than light intensity 35 at the time of the optical focus state. However, while minimum value 32c is larger than minimum value 14c at the time of the optimal focus state, there is no fluctuation in position.
Due to the influence of the abovementioned diffracted light, light intensity 36 at the end of line pattern 10 facing wide space 12, is larger by 3 than light intensity 37 at the opposite end. Similarly, light intensity 38 at the end of dot pattern 11a facing space 12, is wider by 4 than light intensity 39 at the opposite end. However, light intensities 40 and 41 at the end of dot pattern 11b facing narrow space 13 are stable at values almost equal to that of light intensity 39. The light intensity takes the maximum value at the center of each space. However, because of the abovementioned diffraction phenomenon, maximum value 42 in space 13 is smaller than maximum value 43 in space 12.
The light intensity where the exposure process is in the defocusing state greatly fluctuates as compared with that in the case of the optimal focusing state. The light intensity shifts especially in the peripheral part of an area where, among the plurality of identical patterns, patterns whose density have greatly changed are arranged.
FIG. 3C shows a pattern shape of the photoresist when the exposure is carried out with the light intensity shown in FIG. 3B. The photoresist is a positive type. As shown in FIG. 3C, processed film 45 is formed on semiconductor substrate 44 that is a part of the wafer, and the photoresist that becomes a processed mask of processed film 45 is formed on processed film 45. Then, the patterns of photomask 500 are transferred to the photoresist.
The patterns of the photoresist that becomes the processed mask are largely classified into line resist 46 and dot resist 47. Dot resist 47 is further classified into dot resist 47a adjacent to line resist 46 via space 48, and dot resist 47b adjacent to dot resist 47a via space 49.
Since the photoresist is the positive type, the photoresist at the exposure place is removed by the development process to form spaces 48 and 49. Light intensity at the end facing space 48 increases due to diffracted light, and hence line resist 46 is formed smaller by width X5 than virtual end 50 when there is no diffracted light. Due to the influence of shifted light intensity 34, line resist 46 becomes further smaller by X6. As a result, line resist 46 is formed smaller by (X5+X6) than line pattern 10. Similarly, dot resist 47a facing space 48 is formed smaller by (X7+X8) than virtual end 51. On the other hand, a longest part of dot resist 47b in the X direction is formed with a width almost equal to that of dot pattern 11b shown in FIG. 3A.
As described above, in the case of the defocusing state, there is a possibility that the patterns of the peripheral part of the area in which the plurality of identical patterns are formed may be formed much smaller than those in the case of the optimal focusing state. Thus, the patterns formed by transferring the patterns to the processed film are peeled more easily than that in the case of the optimal focusing state. When pattern peeling occurs, product yield is further reduced.
To prevent smaller formation of the patterns than desired shapes, there has been disclosed a method for arranging a dummy pattern. JP2010-191403A (hereinafter, Patent Literature 1) discloses formation of an auxiliary pattern symmetrical to a main pattern. JP2008-116862 (hereinafter, Patent Literature 2) discloses formation of a projection in an auxiliary pattern close to the leading end of a main pattern. JP 2007-194492 (hereinafter, Patent Literature 3) discloses arrangement of a dummy pattern having a projection at an extension part of a repetitive pattern (memory cell array). JP 2000-122263 (hereinafter, Patent Literature 4) discloses a mask pattern having a projection equal to or less than a resolution limit.
In the method disclosed in any one of Patent Literatures 1 to 4, in a case where defocusing occurs in repeatedly arranged dot patterns, the dot patterns may be deformed to be peeled even when dummy patterns are arranged. The peeled patterns cause the problem of a reduction in product yield.